Stress strain curve is behavior of material when it is subjected to load. In this diagram stresses are plotted along the vertical axis and as a result of these stresses, corresponding strains are plotted along the horizontal axis. Shown below in the stress strain curve foe mild steel.

From the diagram one can see the different mark points on the curve when a ductile material like mild steel is subjected to tensile test, then it passes various stages before fracture. They are:

PROPORTIONAL LIMITProportional limit is point on the curve up to which the value of stress and strain remains proportional. From the diagram point P is the called the proportional limit point or it can also be known as limit of proportionality.Hook’s law is applicable up to this point.ELASTIC LIMITElastic limit is the limiting value of stress up to which the material is perfectly elastic. From the curve, point E is the elastic limit point. Material will return back to its original position, If it is unloaded before the crossing of point E.UPPER YIELD POINTYield stress is defined as the stress after which material extension takes place more quickly with no or little increase in load. Point Y is the yield point on the graph and stress associated with this point is known as yield stress.LOWER YIELD POINTAfter the yield point, the curve typically decreases slightly because of dislocations escaping from Cottrell atmospheres.

ULTIMATE TENSILE STRESS POINTUltimate stress point is the maximum strength that material have to bear stress before breaking. It can also be defined as the ultimate stress corresponding to the peak point on the stress strain graph. On the graph point U is the ultimate stress point.

BREAKING STRESSBreaking point or breaking stress is point where strength of material breaks. The stress associates with this point is known as breaking strength or rupture strength. On the stress strain curve, point B is the breaking stress point. As brittle materials fail due to shear, it will be equal to the ultimate shear stress.TOUGHNESSToughness can be determined by integrating the stress-strain curve. It is the energy of mechanical deformation per unit volume prior to fracture.RESILIENCEThe area under the stress-strain curve upto the elastic limit depicts the Modulus of resilience(MR) which signifies the ability of material to store or absorb energy without permanent deformation.YOUNGS MODULUSYoung's modulus is the ratio of stress to strain(slope of the curve) up to P.​